The Retina Revolution: Signaling Pathway Therapies, Genetic Therapies, Mitochondrial Therapies, Artificial Intelligence

Edward H. Wood; Edward Korot; Philip P. Storey; Stephanie Muscat; George A. Williams; Kimberly A. Drenser


Curr Opin Ophthalmol. 2020;31(3):207-214. 

In This Article

Genetic Therapies

Genetic mutations typically cause retinal disease by forming a protein with decreased or absent function, forming a protein that acquires a new detrimental role, or failing to form a protein at all. Genetic testing is the fundamental first step in the diagnosis and treatment of inherited retinal disease (IRD). The field of genetic testing has made considerable progress from when Watson and Crick[25] first proposed the double helix in 1953. We are now able to perform next-generation sequencing (sequencing huge numbers of samples at once) through multiple service providers[26] to identify mutations and label their pathogenicity based on reference genomes. There are two rapidly advancing therapeutic approaches to treat a subset of retinal diseases: gene therapy and/or gene editing.

Gene therapy is best thought of as gene supplementation. This typically involves encoding the wild-type DNA sequence of a target gene into a small and circular 'plasmid' packaged within a delivery vector. Although there are a wide variety of mechanisms to introduce an engineered plasmid into the cell, the delivery vector is often a recombinant virus (such as adeno-associated virus (AAV) or lentivirus).[27] Once within the cell, the plasmid expresses the wild-type DNA, thereby generating a 'normal' protein to supplement and/or replace the 'abnormal' protein (Figure 1).

Figure 1.

A plasmid containing the wildtype copy of the gene of interest is introduced into a diseased cell. Following viral transduction, the plasmid is read and the gene product is shuttled to the site of interest as needed.

Although there a wide variety of gene therapy trials currently underway for both inherited[28] and acquired retinal diseases (such as age-related macular degeneration[29]), the only FDA approved gene therapy for the eye is Voretigene neparvovec-rzyl (Luxturna, SPARK Therapeutics) for the treatment of Leber congenital amaurosis (LCA) caused by mutations in the RPE65 gene. Luxturna is an AAV-2 delivery vector encasing a plasmid encoding wildtype (WT) RPE65 that is delivered through a subretinal injection.[30] Without treatment, children with LCA due to biallelic RPE65 mutations usually progress to complete vision loss by the third or fourth decade of life,[31] but with treatment (approved for ≥12 months of age in each eye ≥6 days apart), patients experienced improved vision-based navigation as measured by multiluminance mobility testing.[32]

Although the FDA approval of Luxturna in 2017 was a victory for retina and the field of medicine at large, retinal gene therapy has several limitations in its current form. The RPE65 gene is one of the hundreds of (known) genes that lead to IRDs, accounting for an estimated 2% of cases. Furthermore, gene therapy does not treat dominant-negative mutations and is therefore typically limited to addressing autosomal recessive mutations.[33] Although the treatment effect of luxturna appears to last at least 3–5 years based on Phase 1 studies, the exact duration remains unknown.[34] It also appears that retinal degeneration continues to progress in the presence of treatment, albeit much less so in very young patients.[35] Optogenetics overcomes some of these issues, wherein light-sensitive proteins are introduced to cause well defined cellular events in the presence of light.[36] In the field of retina, light-sensitive proteins are introduced into retinal neurons that have no intrinsic light sensitivity thereby imparting light-sensitivity to more downstream retinal elements when photoreceptors and/or other retinal neural elements are damaged.[37] However, optogenetics has several of its own limitations including distortion of the visual experience when downstream retinal elements initiate the signaling cascade and the need for high-intensity light stimulation.[38]

Gene editing comprises two fundamental steps: the creation of double-stranded DNA (dsDNA) breaks at specific locations and dsDNA break correction with gene correction and/or introduction.[39] There are numerous gene-editing technologies available including the widely investigated clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated systems (Cas), zinc-finger nucleases, and transcription activator-like effector nucleases (TALENS).[40] Gene-editing technologies may be delivered similarly to gene therapy. CRISPRs interface with Cas to form an RNA-guided protein complex that recognizes a component in the target DNA (protospacer adjacent motif) and cleaves at the target nucleic acid sequence[41] (Figure 2). CRISPR–Cas can be used to correct a mutated DNA sequence, induce the expression of certain genes, knock down the expressed of others, introduce foreign DNA into the genome, and modify epigenetic DNA changes.[42]

Figure 2.

CRISPRs associate with CRISPR-associated systems (Cas) to cleave targeted DNA. The most commonly used CRISPR–Cas9 system employs a guide RNA. A component in the target DNA (protospacer adjacent motif) is required for Cas9 to recognize and cleave at the target DNA location.

CRISPR–Cas systems have proven efficacy through ex vivo treatment of induced pluripotent stem cells (iPSCs) derived from patients with inherited retinal diseases[43,44] and animal models of retinal degeneration.[33,44–46] Allergan and Editas Medicine are currently enrolling patients into the Brilliance Phase 1/2 clinical trial of AGN-151587 (EDIT-101), a CRISPR-based gene editing therapy for LCA10 caused by mutations in the CEP290 gene.[46] Of note, this trial is the world's first in-vivo study of CRISPR-based gene editing in medicine. An ongoing concern with CRISPR–Cas is off-targeting effects which can result in unintended deleterious mutations, but significant progress is being made to increase specificity.[43]